Ultraviolet light is known to cause various changes in skin. In dermatology, the wavelength spectrum is divided into the long wavelength ultraviolet light region, i.e. 400-320 nm, the medium wavelength ultraviolet light region, i.e. 320-290 nm, and the short wavelength ultraviolet light region, i.e. 290 nm or less. These regions are referred to as UV-A, UV-B and UV-C, respectively.
Usually, most of the ultraviolet light which people are exposed to is from sunlight. The part of the ultraviolet light which reaches the earth comprises UV-A and UV-B, while UV-C is absorbed by the ozone layer and very little of it reaches the earth. Of the ultraviolet light which reaches the earth, UV-B is known to cause erythema and blisters and accelerate formation of melanin when the skin is irradiated by more than a certain quantity, while UV-A is known to darken the skin immediately after the irradiation and also to enhance the skin-degradating action of UV-B. UV-A is important in terms of prevention of the development and aggravation of liver spots and freckles.
However, research on the action of UV-A on the skin started relatively recently, and only dibenzoylmethane derivatives are known as UV-A absorbents. Benzophenone derivatives are known as UV-AB absorbents which have absorption bands covering both UV-A and UV-B regions.
For the base for endermic liniments, silicone-type bases such as dimethylpolysiloxane are widely used. This is largely because the silicone-type bases are easy to use, i.e. they spread well, give a refreshing feeling and are not sticky, and also because of their superior functioning, i.e. they are not washed away easily by perspiration or water.
Flowever, the existing UV-A, UV-B and UV-AB absorbents have very low compatibility with the silicone-type bases. Therefore, in order to blend them into an endermic liniment with the silicone-type bases, it was necessary to add an oil base. This made it difficult to fully utilize the advantages of the silicone-type bases described above.
Due to this problem, development of an ultraviolet absorbent which is soluble in silicone oil and superior in terms of water resistance and oil resistance, and at the same time gives sufficient protection against ultraviolet light, has been strongly desired.